Field of the invention
The present invention relates to a refractive index distribution structure and a manufacturing method of the same, and an image display apparatus having the refractive index distribution structure. In particular, the present invention relates to a refractive index distribution structure for use in a light diffusion plate and a manufacturing method of the same.
Description of the Related Art
The refractive index distribution structure including a background medium and a plurality of elements, having a refractive index different from that of the background medium, which is contained in the background medium, is used as various optical functional devices such as a photonic crystal and a light diffusion plate.
When light is incident on the refractive index distribution structure, the angular characteristics of the scattered light transmitted or reflected therein are as follows. When the elements are arranged periodically, the angular characteristics are such that the scattered light intensity in a specific direction corresponding to each diffraction order is strong due to interference of lights diffracted by the elements. Meanwhile, when the elements are arranged in a completely random manner, the angular characteristics of light-scattering are uniform. In some cases, e.g. when the structure is used for screen, the angular characteristics of light-scattering need to be appropriately controlled. In this case, if the angular characteristics of the light-scattering are to be controlled, the element shapes should be made uniform, the elements should be randomly arranged, and the average value of each inter-element distance (distance between elements) and the variation from the average value should be appropriately managed.
As a method of manufacturing an appropriately controlled random refractive index distribution structure, there has been known a method of forming a microsphere-dispersed refractive index distribution structure (H. W. Deckman et al. “Natural Lithography,” Appl. Phys. Lett. vol. 41, No. 4, pp. 377, (Aug. 15, 1982)). Such a refractive index distribution structure is configured to manage the average distance between microspheres such that the number of microspheres dispersed per unit area is made smaller than the number of microspheres required for closest packing.